Problems with the Voltage Regulation Module VRM

[juchong]

Hey everyone,

We had an issue with one of the VRMs on our test robot last week where the radio would intermittently drop the driver station, but maintain communication with our debug PC. We're using LabVIEW on our robots this year.

The DS would drop while idling, moving, enabling, etc. which seemed odd. In past years teams would have issues with communication dropping because of a loose cable, a bad connection, bad soldering, etc. but I can't ever recall seeing a bad voltage regulation module at a competition.

I swapped radios and still experienced the same issue, so I decided to remove the VRM and replace it with last year's module (remember, this is a test robot).

I finally had a chance to test the module in the lab and found quite a few issues with it. I used a high power decade resistor box as my load, a calibrated Keithley power supply set at 12.3V/1.5A as my source, a calibrated Fluke multimeter to verify the resistance of the decade resistor box, and a calibrated Tek oscilloscope to capture the waveforms.

TEST 1: 5 V / 2A Output
Load: 3 ohms (measured) - Calculated current: 1.6A

• If the load is attached to the power supply while powering up the VRM, the VRM will not start up.
• Powering on the VRM with no load, then attaching the load will allow the VRM to stabilize.
• Ripple on the supply was hideous; about 5.4V p-p. This would definitely explain why our radio was rebooting.

Screenshot of test 1's waveform. Note that the oscilloscope is set to AC coupling.

Test 2: 5 V / 2 A Output
Load: 4 ohms (measured) - Calculated current: 1.25A

• If the load is attached to the power supply while powering up the VRM, the VRM will not start up. Same as the previous test.
• Powering on the VRM with no load, then attaching the load will allow the VRM to stabilize.
• Ripple on the supply was hideous; about 4.2V p-p. This would definitely explain why our radio was rebooting. It's also important to note that the maximum rated power draw from the radio is 1A, not too far from these test conditions.

Screenshot of test 2's waveform. Note that the oscilloscope is set to AC coupling.

Test 3: 5 V / 2 A Output
Load: 10 ohms (measured) - Calculated current: 500mA

• The VRM will stabilize after about 5 seconds and correctly output 5V on both supplies, but it will draw 1.5A (current limited) from the 12.3V supply. That's one heck of a start-up transient.
• Powering on the VRM with no load, then attaching the load will allow the VRM to stabilize quickly.
• Ripple on the output is still bad. 1.1V p-p. At such a low power rating, this is unacceptable. Ripple should be on the order of 50 ~ 100mV maximum for a bad supply, 10 ~ 30mV for a high quality supply.

Screenshot of test 3's waveform. Note that the oscilloscope is set to AC coupling.

Conclusion
It looks like there is something seriously wrong with this module. I'm planning on taking similar data on another VRM in order to compare the results. I really hope we don't have to walk around with oscilloscopes at champs!